1. Field of the Invention
This invention relates to homogeneous liquid-phase catalytic oxidation processes and to catalysts used in these processes.
2. Discussion of the Background
Catalysts are substances which facilitate reactions. At a given temperature, a catalyst accelerates the rate of a reaction. The term "catalyst" in this document is used in accordance with this standard meaning.
Catalysis is broken down into two different and unrelated classes of catalysis: (1) heterogeneous catalysis; and (2) homogeneous catalysis. In heterogeneous catalysis, the reaction mixture contains materials in at least two different physical states. Generally, the catalyst is in the form of a solid, and the substrate and product are in the form of either liquids and/or gases. Heterogeneous catalysis is characterized by high temperatures, high pressures and lower selectivity. This is the form of catalysis typically used in large-scale industrial applications (e.g., oil refining and coal liquefaction).
In homogeneous catalysis the reaction mixture is essentially made up of one physical phase. The catalyst, the substrate and the product are all typically dissolved in the reaction mixture. Homogeneous catalysis is characterized by lower temperatures, lower pressures and higher selectivity as compared to heterogeneous catalysis. It thus requires less energy and can be used on more sensitive substrates which could not withstand the temperature and pressure regiment of heterogeneous catalysis.
Because of the very nature of heterogeneous catalysis and homogeneous catalysis, catalysts used in heterogeneous catalysis and catalysts used in homogeneous catalysis are considered to be two distinct and different systems in the art of catalysis. A heterogeneous catalyst is essentially always an insoluble solid material designed to withstand the adverse environment of heterogeneous catalysis without loss of its catalytic activity. A heterogeneous catalyst is designed to remain insoluble in the reaction medium to facilitate separation of the catalyst from the rest of the reaction mixture upon completion of the reaction.
Heterogeneous catalysts are typically inorganic materials selected on the basis of their ability to remain insoluble in the reaction medium, and withstand high temperatures and high pressures without change. Examples of typical heterogeneous catalysts include zeolites and transition metals supported on an inorganic matrix.
By contrast, homogeneous catalysts are materials which are selected because of their ability to dissolve in the reaction medium where they exhibit catalytic activity. Homogeneous catalysts are typically materials which, at a molecular level, possess a large organic component. Transition metals coordinated to various organic ligands are typical homogeneous catalysts. Homogeneous catalysts are thus selected on the basis of their ability to dissolve in the reaction medium and to exhibit selective catalytic activity.
Since only a limited number of elements are available, one will, of course, be able to find structurally similar materials used both in heterogeneous and homogeneous catalysts. But this overlap is only accidental, and, in the art of catalysis, a material's catalytic activity in a heterogeneous catalytic environment does not suggest its use in a homogeneous catalytic system, and vice a versa.
The partial oxidation of organic substrates using either homogeneous or heterogeneous catalysts is one of the most important processes used to transform organic substrates into desired materials or intermediates used in the production of desired materials. For example, the partial oxidation reaction of hydrocarbons is one of the most applied processes for converting hydrocarbons into valuable chemical intermediates.
Typical examples of homogeneous catalyst catalyzed oxidation reactions include the partial oxidation of hydrocarbons: (1) the oxidation of carbon-hydrogen bonds (e.g., alkanes)) to the corresponding alcohol, also known as the carbon-hydrogen bond (e.g., alkane) hydroxylation reaction; and (2) the epoxidation of alkenes.
A not so considerable but substantial and growing amount of work has been done on the catalytic homogeneous oxidation of alkanes to obtain the corresponding alcohol (this process is also known as alkane hydroxylation or carbon-hydrogen bond hydroxylation), illustrated in equation (1) below: ##STR1##
Considerable work has been done on the catalytic homogeneous epoxidation of alkenes, illustrated in equation (2) below: ##STR2##
The most established technology in the homogeneous epoxidation of alkenes is the epoxidation of olefins by alkyl hydroperoxides catalyzed by mononuclear early transition metal complexes. These epoxidation reactions use typically titanium, vanadium, and molybdenum species. See for example, Aldrichimica Acta, Vol. 12, No. 4, pp. 63-73 (1979) summarizes much of this type of oxidation reaction. J. Org. Chem., 1986, 51, 1922-1925 and Chemical and Engineering News, page 24, June 2, 1986 specifically focus on the application of epoxidation reactions to chiral epoxidation. These early-transition/alkylhydroperoxide epoxidation processes have not been observed to be active for the activation and functionalization of alkanes.
Metalloporphyrins and high valent oxometalloporphyrin intermediates include biological materials which function as homogeneous catalysts capable of attacking both alkenes and alkanes. One of these biological materials, cytochrome P-450, is the most potent oxidant of organic molecules in the biosphere. This enzyme can utilize dioxygen plus electrons to oxidize substrates, or like the metalloporphyrins and other synthetic active site analogs, cytochrome P-450 can utilize oxygen donors to oxidize substrates.
The principal academic work on the homogeneous catalytic alkane functionalization systems appeared in several papers, principally in 1983. Two principal papers are: J. A. Smegal and C. L. Hill, J. Am. Chem. Soc., 1983, 105, 3515 and J. T. Groves and T. E. Nemo, J. Am. Chem. Soc., 1983, 105, 6243. The Groves paper addresses the Fe.sup.III TPP(X)/iodosylbenzene system (TPP=the tetraphenylporphyrinato dianion ligand). The Smegal paper gives a fairly complete discussion of the mechanism of catalytic alkane functionalization by the MnTPP(X)/iodosylbenzene system.
All of the homogeneous catalytic oxygen atom transfer oxidation systems discussed above rely on the use of catalysts that contain organic ligands. Inasmuch as all organic ligands are inherently unstable with respect to oxidation by dioxygen as well as to oxidation by strong oxidizing agents which include oxygen donors, the oxidative instability of these catalysts has been the limiting factor to their success in these processes.
In the epoxidation processes, and particularly in alkane hydroxylation processes, oxidative degradation renders homogeneous catalysts inactive after as little as 50 turnovers. (A turnover is one interaction between the catalyst and a substrate molecule to produce a product molecule.)
Prior to the present invention the most stable catalytic system for homogeneous catalytic oxygen atom transfer oxidation of hydrocarbons via oxometal intermediates was the system constituted by the oxidatively resistant metalloporphyrin, tetrakis(2,6-dichlorophenyl)porphinato-iron(III) chloride (FeTDCPPC1) and the highly reactive oxidatively resistant oxygen donor, pentafluoroiodosylbenzene (PFIB). These systems which are disclosed and discussed in Traylor et al., J. Chem. Soc., Chem. Commun., 1984, 279 are however still not stable enough to permit their use in an economic setting.
Thus, in view of the numerous advantages inherent to homogeneous catalysis which is unfortunately limited by the stability of the catalytic materials used therein, there exists a strongly felt need for a new process for the homogeneous catalytic oxidation of organic substrates. Such a new process should ideally be based on a catalyst which would provide the advantages of homogeneous catalysis, e.g., selective oxidation of organic substrates at low temperatures and low pressure requirements, without suffering from the drawbacks which have heretofore limited homogeneous catalysis, especially including catalyst sensitivity to the reaction medium and conditions.